Practical Medical Physics Session: TG-151amos3.aapm.org/abstracts/pdf/77-22539-313436-92755.pdf ·...

transcript

Practical Medical Physics

Session: TG-151

Dose Monitoring

August 5, 2013

Katie Hulme, M.S.

Digital Imaging and ‘Dose Creep’

Under-Exposed Over-Exposed

Images courtesy of Agfa Healthcare©

Freedman et al., The potential for unnecessary patient exposure from the use of storage phosphor imaging systems, SPIE Medical Imaging,

SPIE Proceedings 1897, 472-479 (1993)

Gur et al., Natural migration to a higher dose in CR imaging, Proceedings of the Eight European congress of Radiology, 154 (1993)

Dose Tracking – Annual

(Physicist)

• Tube Output, HVL

• Incident Air Kerma (Ka,i) Measurements - ‘typical’ doses

- references for limits / reference levels:

• NCRP 172

• NEXT Surveys

• State regulations

• AEC evaluation - EI is useful for this as well!

- TEIs will be correlated w/ cutoff dose

• Accuracy of metric used for ongoing QC - DAP, EI, etc.

CCF Patient Incident Air

Kerma (IAK) • GOAL:

- to reduce patient doses for common radiographic

exams to below 3rd quartile NEXT* data for ALL

*NEXT = National Evaluation of X-Ray Trends( CRCPD Pub. No. E. 03-2)

Where we were…

EXCEEDING NEXT 3rd

QUARTILE

Where we were…

kVp Bin

AP Abdomen

Frequency

EXCEEDING NEXT 3rd

QUARTILE

CCF Patient Incident Air

Kerma (IAK) • HOW:

- kVp standardization for select exams • Enables comparison of IAKs between sites with same system

- Development and documentation of image-based

methodology for in-house AEC evaluation and

calibration

- Instituted new CCF limit for IAK • Identify outliers during annual testing

*NEXT = National Evaluation of X-Ray Trends( CRCPD Pub. No. E. 03-2)

CCF IAK Limits

Min Max Q3 Q3 Av Av Limit

(mGy) (mR) (mGy) (mR) (mR) (mR) (mGy) (mR) (mGy) (mR) (mR)

AP Abdomen 5.26 600 3.40 388 300 490 3.469 396 2.374 271 450

AP Lumbar 6.13 700 4.20 479 4.179 477 2.996 342 550

AP Thoracic 3.50 400 2.27 325 325

AP Cervical 1.75 200 1.75 200 1.183 135 120

LAT Skull 1.75 200 1.75 200 1.270 145 150

DP Foot 0.88 100 0.31 35 8 35 50

PA-AP Chest w/ Gr 0.35 40 0.26 30 10 15 0.158 18 0.114 13 30

PA-AP Chest woo Gr 0.26 30 0.18 20 0.123 14 0.079 9 20

PA-AP Chest w/ Gr 0.35 40 0.26 30 10 15 0.158 18 0.114 13 30

NEXT DataODH

CRCPD, Pub No. E-03-2, Table 4

NEXT DataCCF ESE

Standard

ESE Range

quoted by

NEXT = National Evaluation of X-Ray Trends

CRCPD = Conference of Radiation Control Program Directors

References: Diagnostic Reference

Levels (DRLs)

• NCRP Report No. 172, Reference levels and achievable doses in medical and dental

imaging: recommendations for the United States. (2012)

• ACR Practice Guideline for Diagnostic Reference Levels in Medical X-Ray Imaging.

(Revised 2008, Resolution 3).

• Gray et al., Reference Values for Diagnostic Radiology: Application and Impact,

Radiology Vol 235 (2):p354-358, 2005.

• Nationwide Evaluation of X-ray Trends (NEXT): Tabulation and Graphical Summary

of 2002 Abdomen/Lumbosacral Spine Survey. CRCPD Publication E-06-2b (2006).

• Nationwide Evaluation of X-ray Trends (NEXT) : Tabulation and Graphical Summary

of 2001 Survey of Adult Chest Radiography. CRCPD Publication E-05-2 (2005).

• Nationwide Evaluation of X-ray Trends (NEXT) : Tabulation and Graphical Summary

of 1998 Pediatric Chest Survey. CRCPD Publication E-04-5 (2004).

Ka,i - Limitations

• ‘Average’ patient doses do not necessarily

reflect actual patient dose or the distribution

in patient doses - Measurements do not indicate adherence to technique

charts (manual)

• Phantoms represent a limited range of exam

types and body parts

• Metrics are not suitable for ONGOING QC - Require a level of expertise (and equipment) to measure

Dose Tracking – Ongoing

Choose a Metric

Verify Indicator Accuracy

Establish Target Values

Education Education Education

Develop Action Criteria

Reporting Mechanism

• Exposure Index

- DICOM tags*: EI(0018,1411), TEI (0018,1412), DI (0018,1413)

- Available for all systems that have adopted IEC standard

• Entrance Dose

- DICOM tags:

• Entrance Dose (0040,0302)

• Entrance Dose in mGy (0040,8302)

- Available on systems with integrated generator

• Area Dose Product

- DICOM tag:

• Image and Fluoroscopy Area Dose Product (0018,115E)

- Available on systems with integrated generator

WHAT INFORMATION IS AVAILABLE TO YOU??

*DICOM Correction item 1024 – ‘Exposure Index Macro’

Exposure Index (IEC 62494-1 )

)(0 VgcE I

• Where

- V is the Value of Interest

- g(V) is the inverse calibration function

- C0 = 100 µGy-1

Exposure Index

• Advantages - Reflects receptor dose

- Not as dependent on patient size/distribution

- Standardized metric

• Disadvantages - Indirectly related to patient dose

- Depends on beam quality, exam/view, as well as vendor-

defined VOI

- Collimation, prosthetics, etc. can affect calculated value

Entrance Dose

• Incident air kerma (Ka,i) at a fixed location

- Reference point varies among vendors

• Typically derived from exam parameters

• kVp / mAs

• not measured on a patient by patient basis

Entrance Dose

• Advantages - Can be used to estimate patient dose

• Disadvantages - No standard reference point or method for normalization

- Entrance surface of patient may deviate from reference point

- Does not represent size of the x-ray field

• Most data from Europe - But often limited to certain body habitus range

• i.e. 65-75 kg, Hart 2003

- Most US data currently w/ respect to phantoms

Image and Fluoroscopy Area

Dose Product

• Product of the x-ray field size and air kerma

- Dose Area Product (DAP)

- Kerma Area Product (KAP)

- Air Kerma-Area-Product (PKA)

- Unit DICOM field: dGy-cm2

• Often measured using a PKA meter installed

on the collimator

• Advantages

- Contains information about Ka,i AND field size

• Enables assessment of both patient dose and

collimation

- Field size can be derived if Ka,i is known (or

estimated)

• Disadvantages

- DAP meter option may have to be purchased

separately

- Difficult to isolate impact of collimation without

knowledge of Ka,i

Choose a Metric

Reporting Mechanism

• IEC 62494-1 standard states that the

EI shall be calibrated such that:

• Where

- KCAL is the receptor air kerma (in µGy)

under calibration conditions

- C0 = 100 µGy-1

C A LKcE I 0

• Inverse calibration function is

defined as:

• Inverse calibration function should

have an uncertainty of less than 20%

)()( 1

C A LC A LC A L VfVgK

Calibration Conditions (IEC 62494-1 )

• Fixed radiation quality

- RQA5

• Homogenous irradiation of image

receptor

• Measurement of incident air kerma

(free in air, no backscatter)

• Value of Interest (VOI) calculated

from central 10% of image area for

flat field images

Clinical Experience….

• 80 CR readers (Agfa)

• 38 units required PMT replacement

(~50%)

0.25 0.

55 0.7

0.85 1

1.15 1.

45 1.6

1.75 1.

Sensitivity

PRIOR TO

TESTING:

Mean = 0.853

SD = 0.233

Compliant

Prior to testing:

Post QC and Repair:

0.25 0.

55 0.7

0.85 1

1.15 1.

45 1.6

1.75 1.

Sensitivity

POST QC AND

REPAIR:

Mean = 0.971

SD = 0.099

Compliant

Variation in Sensitivity

Exposure Indicator Accuracy

(computed radiography)

• How well matched should my readers

- ±25% should be achievable

- TG-10 recommends readers be matched

within ±10%

- Can adjust the high-voltage settings on

some units

- In other cases have to replace the PMT

Indicator Accuracy

• EI

- ± 20% - IEC 62494-1

• PKA

- ± 35% - IEC 60601-2-43

- For PKA > 2.5 Gy-cm2

• Ka,i

- Vendor-defined

Choose a Metric

Reporting Mechanism

• DI is only useful if you have selected a

reasonable TEI

• Some vendors will provide recommended

TEI values

Establishing TEI Values

DR MANUFACTURERS

AEC Sensitivity Calibration

kVp Grid? Phantom Target Ka

(µGy)

GE Flashpad (CsI) 80 No 20 mm Al 2.5

Siemens (CsI) 70 No 0.6 mm Cu 2.5

Agfa DX-D (CsI) 70 No 25 mm Al 2.5

Philips 70 No 25 mm Al 2.5

Carestream DRX1-C 80 -- 0.5 mm Cu + 1.0 mm Al 2.5

Canon CXDI-70C 80 Yes 20 cm PMMA 2.5

• Can calculate expected EI or PV for target Ka under

AEC calibration conditions

AEC Calibration and EI

• VOI can matter

- Make sure to use the appropriate exam

- Know the VOI used for EI calculation

• If using a target EI:

- Must verify accuracy of exposure

indicator and account for it

- For CR

• Time between image and readout must be

kept consistent

• Use QC plate or plate of median sensitivity

• The fewer sub-groups you have, the easier

your TEI values are to implement…

• Our Agfa CR systems currently set up with

three TEI sub-groups

• But are these right?

• Chest (TEI – 350)

• Non-Extremity (TEI – 400)

• Extremity (TEI – 1000)

Establishing TEI Values

Entrance Air Kerma

• Still requires establishing a target value for it

to be useful for ongoing QC

- Individual values extremely dependent on patient

- No standardized method for normalization

- Sample mean/ median < a DRL

• DRL specific to Exam

• DRL should be adjusted to account for patient

distribution OR

• DRL evaluation should be limited to specific

weight category (difficult to automate this!)

Choose a Metric

Reporting Mechanism

Setting Action Criteria

• Shape of distribution

• Expected variation

EI - Patterns

• Typically, 95% within +/- 2 DI

• SD in EI increases when manual techniques

are used

• Log-normal distribution of EI

• Normal distribution of DI

- SD in DI is independent of TEI

• Guidelines yet to be published

• Questions still to be answered:

- What is a typical (acceptable) level of variation in

the EI and DI

- Are recommended TEI values optimized?

Hulme et al, A Method for Deriving Exam-Specific Target Exposure Indices

(TEI) in Computed Radiography as a Function of a Reference TEI, TU-A-116-4

Color Coded Exposure Bar Ranges

- Green (Go)

• DI between -3 to +3 deviation units (aim 0)

• Represents less than a 2x change (±) in exposure index from target

• Images should be acceptable for exposure (no additional review required)

- Yellow (Caution)

• DI between -6 and -3 or +3 and +6 Deviation Units

• Represents a 2x to 4x change (±) in exposure index from target

• Images may be under or overexposed, but could still be acceptable for use

• Further review with supervision may be required to determine if repeat is

needed

- Red (Alert)

• DI < -6 or > + 6 deviation units

• Represents a greater than 4x change (±) in exposure index from target

• Images are probably significantly under or overexposed and are not acceptable

• Technique settings and targets should be checked

• Images should be reviewed with supervision and repeated (as needed)

Color Coded Exposure Bar Ranges

Table provided courtesy of Agfa HealthCare.

Choose a Metric

Reporting Mechanism

• EI = Exposure Index

• Approximate exposure to the plate

• LINEAR with exposure

• Double the mAs, EI doubles

• TEI = Target Exposure Index

• ‘Ideal’ exposure to the plate

• DI = Deviation Index

• How far above/below the TEI you are

IEC Exposure Index

Deviation Index (IEC 62494-1 )

• A DI of 0 indicates the exposure was at the

target value

• ±1 DI = ~ ±25% difference in exposure, or

+1/-1 density on a phototimer

• +3 DI = 2x the target exposure

• -3 DI = ½ the target exposure

E ID I lo g10

TG-116 Recommendations

Deviation Index

Exposure Deviation Index - DI Correction Needed

Over Exposed 6 Repeat if Image Saturated

Reduce mAs 0.25x*

>3 Caution

Decrease mAs 0.5x*

2 None

1 None

Aim 0 None

-1 None

-2 None

<-3 Possible Repeat

Increase mAs 2x*

Under Exposed -6 Repeat

Increase mAs 4x*

*If needed based on image quality or dose

Table provided courtesy of Agfa HealthCare.

General Radiography IV March 2011 47

Exposure Index

Deviation Index

75 kVp, 6.3 mAs

@Target Exposure:

EI = 389 (~ 400)

DI = -0.1 (~ 0)

Screen shot courtesy of

Agfa HealthCare

General Radiography IV March 2011 48

Exposure Index

Deviation Index

75 kVp, 3.2 mAs

½ mAs:

EI = 204 (~ 200)

DI = -2.9 (~ -3)

DI > -3 = green

Agfa HealthCare

Exposure Indices

• Remember, clinical exposure indices will

vary with

- Manufacturer (different VOIs)

- Anatomical view

- Collimation

- Exposure indicator accuracy

• Manual techniques will have larger variation

than photo-timed exams

• Errors in detecting collimation borders can

result in inaccurate calculation of EI

- i.e. Merchant view for knees

Choose a Metric

Reporting Mechanism

Data Collection

• Paper (single site)

• Modality Performed Procedure Step (MPPS

report)

• RIS – extract and archive data (DICOM RDSR

or MPPS)

• Send images to a separate server and strip

Multiple Options…… (TG-151)

Data Collection

• Export data from workstation

- Easiest option but not always packaged in a

manner useful to the technologist

- Need the option to export data in both formats

• xml or csv

• SIMPLE report for routine QC

- Accidental or intentional deletion of data can

occur (i.e. during software upgrade by service

engineer)

Multiple Options…… (TG-151)

Defining a ‘Test’ Exam

Export Dose

Monitoring

Statistics

Agfa HealthCare

Choose a Metric

Reporting Mechanism

RE-EVALUATE

Hulme et al, A Method for Deriving Exam-Specific Target Exposure Indices

(TEI) in Computed Radiography as a Function of a Reference TEI, TU-A-116-4

TEI vs. Exam Group

References: Exposure Indices

• Exposure Indicator for Digital Radiography, AAPM Report No. 116, 2009.

• IEC 6294-1, Medical electrical equipment – Exposure index of digital X-ray

imaging systems – Part 1: Definitions and requirements for general radiography,

• Jones et al. ‘One Year’s Results from a Server-Based System for Performing

Reject Analysis and Exposure Analysis in Computed Radiography’, J Digital

Imaging, Vol 24. No 2 (April), 2011: pp 243-255

• Cohen et al. ‘Quality assurance: using the exposure index and the deviation

index to monitor radiation exposure for portable chest radiographs in neonates’,

Pediatr Radiol (2011) 41:592-601

References: Entrance Dose

• Akinlade et al. Survey of dose area product received by patients undergoing

common radiological examination in four centers in Nigeria, J. of Applied Med

Phys Vol. 13, No. 4, 2012: 188-196

• Hart et al., The UK National Patient Dose Database: now and in the future, Br. J.

of Radiol. 76 (2003), 361-65

• Hart et al., UK population dose from medical X-ray examinations, Eur. J. of

Radiol 50 (2004) 285-91

• Meghzifene et al. Dosimetry in diagnostic radiology, Eur. J. of Radiol 76 (2010)

References: PKA

• IEC 60601-2-43, Medical electrical equipment – Part 2-43: Particular

requirements for the safety of x-ray equipment for interventional procedures.

Geneva: International Electrotechnical Commission ed. 2.0, 2010

• Akinlade et al. Survey of dose area product received by patients undergoing

common radiological examination in four centers in Nigeria, J. of Applied Med

Phys Vol. 13, No. 4, 2012: 188-96

• Hart et al., The UK National Patient Dose Database: now and in the future, Br. J.

of Radiol. 76 (2003), 361-65

• Hart et al., UK population dose from medical X-ray examinations, Eur. J. of

Radiol 50 (2004) 285-91

• McParland et al. A study of patient radiation doses in interventional radiological

procedures, Br J Radiol. 1998; 71(842):175-85

• Nickoloff et al., Radiation Dose Descriptors: BERT, COD, DAP, and Other

Strange Creatures, Radiographics, Vol. 28. No. 5, 2008

Practical Medical Physics Session: TG-151amos3.aapm.org/abstracts/pdf/77-22539-313436-92755.pdf ·...

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